A study suggesting that organic agriculture gives better pest control and larger plants than conventional farming is sure to reignite longstanding debates about the merits of organic versus conventional agriculture. It also highlights an often-neglected aspect of biodiversity.
"Organic agriculture promotes more balanced communities of predators," says David Crowder, author of the new study published today in Nature1.
"Our study does not tell farmers they should shift to organic agriculture. What our study suggests is that organic agriculture is promoting these more balanced natural enemy communities and they may have better, organic pest control."
Much focus is put on species numbers or ‘richness’. But the research by Crowder, an insect ecologist at Washington State University in Pullman, and his colleagues, shows the importance of ‘evenness’ — the relative abundance of different species. Evenness quantifies not just the presence of different species, but whether one is dominant or whether there is an equal distribution of numbers between species.
The team looked at the bugs, nematodes and fungi that attack the hated Colorado potato beetle (Leptinotarsa decemlineata).
They conducted a meta-analysis of data collected on these denizens of Washington potato fields and found that although organic and conventional farms did not differ markedly in the richness of beetle eaters, the evenness of predators differed "drastically". Organic fields — where only a limited number of man-made chemicals can be used — had far greater evenness than those where pesticides were applied regularly.
Furthermore, the team set up an experimental field in which they manipulated the evenness of predators. Increasing the evenness led to what the researchers call a "powerful trophic cascade", resulting in fewer potato-munching beetles and larger potato plants.
Although the work of Crowder and his group does not address the issue of yields from organic versus conventional farms, their study found that the increased evenness of organic farms compared with that of conventional farms led to 18% lower pest densities and 35% larger plants. Bigger plants generally mean greater potato yields.